The present invention relates generally to a system for slitting a web, and more particularly to a web slitting system having therein means to simplify the removal of defective portions of the web prior to slitting.
In the packaging industry, it is a common practice to print a packaging design on a web made of a material such as plastic or paper and use the printed web for the outer packaging. It is economical to print a relatively wide web with a design for several packages side by side and then slit the printed web into separate webs which are wound onto rollers and used for separate packaging operations. Sometimes the printed web has another web laminated to it to provide additional strength.
The packager who will be using the printed web wants to avoid making packages with printing defects. Accordingly, it is a common practice in the manufacturing of webs to remove any portion of the web which has a defect prior to the slitting operation. Typically, a first operator visually monitors the printed web right after it has been printed to determine if a defect is present. When the first operator observes a defect, he marks the edge of the web near the defect with a piece of masking tape or the like. This is referred to as "flagging" a defect. Generally, flags are put onto the web forward of the defect. If the printed web is going to have a second web laminated to it, a second operator is needed to observe the flagged defect because the laminating operation would cover the flag (which is on the reverse side of the printed web). After lamination the second operator places a second flag at the edge near the defect. The laminated printed web is wound onto a roll for subsequently slitting in a slitting machine. A third operator observes the laminated printed web as it moves from its roll into the slitting machine and stops the slitting operation when the flag is observed. A significant amount of good salable web is lost by slitter operators who stop the slitting operation at, or even before, a flag and then cut out good material between the flag and the defect, rather than running the slitting operation until the actual defect becomes visible. Alternatively, operators continuing the slitting operation until the defect becomes visible frequently permit a small amount of defective web to pass into the slitting machine before the slitting operation can be stopped. This is a natural result of the limitations of human reaction time and the relatively short distance between the flag and the defect, as well as the various human errors in stopping the system. In order to remove the small amount of defective web run into the slitting machines, it is necessary either to run the web further through the slitting machine and remove the defective web from each of the various slit rolls on which the defective slit web has been wound or to wind the defective web back from the slitting machine and then remove it (the latter alternative requiring several necessary time-consuming preliminaries--for example, the web take-up roll shafts must be raised, the slitter blades must be retracted, the clutch pressure must be reduced, etc.). Both of these techniques for retrieving the defective web from the slitting machine are both time-consuming and web-wasting.
It is estimated that the prior art operations for removing defective portions of a printed web result in a substantial loss of defective-free salable web. In fact, it is estimated that 50% of the web cut out is good and the loss runs about $2.50 per pound. A large commercial operation can have thousands of pounds of waste a year, which represents a significant loss in saleable web.
Thus the need remains for a web slitting system which enables the removal of defective portions of a printed web while wasting only the minimum amount of good salable web and minimizing the downtime for the slitting system while the operator removes the defective portions.
Accordingly, it is an object of the present invention to provide a web slitting system which enables the operator to remove defective web portions while minimizing the loss of good salable defect-free web.
It is another object to provide a web slitting system which enables the operator to remove defective web portions with a minimum of slitter system downtime.
It is a further object to provide a web slitting system which enables the removal of defective web in a manner which minimizes both system downtime and the loss of defect-free web.
Another object of the present invention is to provide slack-inducing apparatus and a slack-inducing method useful in such a web slitting system.